<style title="Hello">
a::before{
  color: red;
  content: "Hello";
}
</style>
<link rel="stylesheet" title="x" href="data:text/css,p%7Bcolor:blue%7D">
<a> world</a>
<script>
  document.styleSheets[0].cssRules[0].style.color='blue';
</script>